Surface acoustic wave devices, comprising a piezoelectric substrate upon which are deposited various configurations of conductive transducers, have found widespread utility in the processing of electronic signals. For example, many television receivers employ such devices as a filter in the intermediate frequency (IF) section of the receiver. Among the advantages offered by SAW filters are relative reduction in the volume of circuitry required to provide a given degree of filtering and improved immunity to spurious electrical noise.
An example of a SAW filter is described in U.S. Pat. No. 4,146,851: "Acoustic Wave Device," assigned to the assignee of the subject invention and hereby incorporated by reference. The filter described therein includes a substantially rectangular substrate that may be constructed from for example, lithium niobate. At a lower corner of one end of the substrate is deposited an input transducer in the form of a series of interdigitated conductive elements. At the upper corner of the opposite end of the substrate is deposited a similary arranged output transducer, also in the form of a series of interdigitated conductive elements. Included as well are a (3db) multistrip coupler and a reflector. The coupler causes signals launched by the transducer to be directed in quadrature (90 degree phase relationship) to both the output transducer and the reflector. The phase relationship between the signals directed to the output transducer and to the reflector result in substantial cancellation of "triple-transit" signals, as thoroughly described in the patent cited supra. A triple-transit signal (or, alternatively, back reflection) can generally be described as an attenuated and time-delayed version of the signal originally launched by the input transducer and intercepted by the output transducer. It is the result of a mechanism whereby signals intercepted by the output transducer are in part reflected back to the input transducer along the filter substrate and subsequently retransmitted back to the output transducer. The triple-transit signal is therefore delayed by a time approximately equal to twice the time required for propogation from the input to output transducer.
The reduction of triple-transit signals is a highly desirable effect because the triple-transit signals result in ripple and other anomalies in the phase and frequency responses of the filter. Ripple in the passband of an TV receiver filter is most saliently manifest as "ghosts" in the image reproduced on the receiver cathode ray tube.
The above explication is intended to convey only some of the complexities inherent in the fabrication of a SAW filter and to indicate the amount of substrate surface required to implement a filter characterized by the desired passband phase/frequency response and out-of-band signal rejection requirements.
A somewhat differently configured SAW filter is described in U.S. Pat. No. 3,872,410: "Surface Wave Filter for TV IF Stage," also assigned to the assignee of the subject invention and hereby incorporated by reference. One of the salient features of that filter is a piezoelectric substrate that assumes a parallelogram form characterized by an acute angle in the range of 20 to 25 degrees. The parallelogram configuration has been found to reduce back reflections propogated by the piezoelectric material. (See Column 4, lines 44 to 52 of that patent.)
Finally, U.S. patent application Ser. No. US81/00452: "Surface Acoustic Wave Filter" also assigned to the assignee of the subject invention, describes a SAW filter having a triangularly configured substrate. The triangular configuration not only reduces by a factor of about one-half amount of substrate material required but also attenuates the degree of triple-transit signal encountered.
The subject invention is directed to a SAW filter that further improves the filters resistance to triple transit signals and improves the rejection of out-of-band signals.